(a) Field of the Invention
The present invention relates to a muffler for internal combustion engine and, more particularly, to a semi-active muffler which is able to enhance engine performance by reducing back pressure without increasing exhaust resistance as well as reduce the noise level of the exhaust of the internal combustion engine.
(b) Description of the Related Art
Mufflers are used to reduce the noise level of the exhaust of internal combustion engines. To increase the muffling effect, exhaust resistance of the muffler must be increased. However, high exhaust resistance reduce engine performance since the exhaust resistance causes the generation of back pressure in the exhaust part of the engine. On the other hand, minimizing exhaust resistance to reduce the back pressure decreases the muffling effect of the muffler. To solve this dilemma, mufflers providing both a high muffling effect and low exhaust resistance is developed.
For instance, Japanese publication No. 97-195749 (Jan. 16, 1996) discloses a muffler provided with a valve which can be adjusted according to the pressure of the exhaust gases. The inside of the muffler is partitioned into an upstream chamber and a downstream chamber by a separator, and an valve is provided in the separator. In this muffler, when the revolving speed of the internal combustion engine is low, the pressure in the upstream chamber is smaller than the sum of the forces applied from a coil spring and the pressure in the downstream chamber such that the valve of the separator is closed. In this case, since the exhaust pressure is low, the pressure does not affect engine performance even with the closing of the valve.
On the other hand, when engine RPM increases and the pressure has increased to a predetermined pressure, the valve of the separator is opened so that the exhaust resistance is decreased.
Also, when the engine RPM is changed from high revolving range to low RPM, the valve is closed. To reduce the impact and noise generated when the valve is closed, a buffer material is mounted around a circumferential edge of a valve opening.
However, the buffer material hardens with time, partly as a result of it being exposed to high temperature exhaust gases. Accordingly, the hardened buffer material loses its shock-absorbing ability such that noise of the valve closing is increased.